perf docs: Document cross compilation

Records the commands for cross compilation with two methods.

The first method relies on Multiarch. The second approach is to explicitly
specify the PKG_CONFIG variables, which is widely used in build system
(like Buildroot, Yocto, etc).

Co-developed-by: James Clark <james.clark@arm.com>
Signed-off-by: James Clark <james.clark@arm.com>
Signed-off-by: Leo Yan <leo.yan@arm.com>
Tested-by: Ian Rogers <irogers@google.com>
Cc: amadio@gentoo.org
Cc: Thomas Richter <tmricht@linux.ibm.com>
Link: https://lore.kernel.org/r/20240717082211.524826-7-leo.yan@arm.com


Signed-off-by: Namhyung Kim <namhyung@kernel.org>